Hey there!

You can say both, but when saying "the" you're referring to the only possible outcome, whereas when saying "a" it means there might be more than one. :)

Both are grammatically correct, but there is a difference between the two:

-"properties of materials are the result of collective behaviors of particles" is better if you are writing either about the properties of materials OR about the behaviors of particles.

-"properties of materials are a result of collective behaviors of particles" is better if you are writing about several things that are a result of different behaviors of particles.

I'd say the first option is more likely to be the correct one in most cases.

Hope this helps!